This disclosure relates generally to surgical navigation systems, and more particularly to a surgical navigation system utilizing magnetoresistance sensors.
Surgical navigation systems track the precise position and orientation of surgical instruments, implants or other medical devices in relation to multidimensional images of a patient's anatomy. Additionally, surgical navigation systems use visualization tools to provide the surgeon with co-registered views of these surgical instruments, implants or other medical devices with the patient's anatomy.
The multidimensional images may be generated either prior to or during the surgical procedure. For example, any suitable medical imaging technique, such as X-ray, computed tomography (CT), magnetic resonance (MR), positron emission tomography (PET), ultrasound, or any other suitable imaging technique, as well as any combinations thereof may be utilized. After registering the multidimensional images to the position and orientation of the patient, or to the position and orientation of an anatomical feature or region of interest, the combination of the multidimensional images with graphical representations of the navigated surgical instruments, implants or other medical devices provides position and orientation information that allows a medical practitioner to manipulate the surgical instruments, implants or other medical devices to desired positions and orientations.
Current surgical navigation systems that include position and orientation sensors, or sensing sub-systems based on electromagnetic (EM), radio frequency (RF), optical (line-of-sight), and/or mechanical technology.
EM sensors are typically implemented with coils or microcoils to generate and detect the magnetic fields. While coil based EM sensors have been successfully implemented, they suffer from poor signal-to-noise ratio (SNR) as the transmitter coil frequency is reduced and/or the receiver coil volume is reduced. Reducing the SNR translates into a reduced range (distance from transmitter to receiver) of the EM sensors that may result in a clinically meaningful position error.
Another problem associated with coil based EM sensors is that they are susceptible to magnetic field distortions that arise from eddy currents in nearby conducting objects. The tracking technique used with coil based EM sensors relies on a stable magnetic field, or a known magnetic field map. Therefore, unpredictable disturbances resulting from metallic objects in the magnetic field reduce the accuracy or may even render the tracking technique useless. Selecting a magnetic field frequency as low as the application allows reduces problems resulting from eddy currents, however it also reduces the sensitivity of coil based EM sensors since these are based on induction.
Other problems associated with coil based EM sensors is that they are generally more difficult and expensive to manufacture and are also inherently sensitive to parasitic inductance and capacitance from the cables, connectors and electronics because the sensor signal is proportionally smaller while the parasitic signal remains the same. While some of the parasitic contributions may be partially nulled out using more expensive components and manufacturing processes, the remaining parasitic inductance and capacitance result in a reduced range.
In addition to coil based EM sensors, there are a large variety of magnetic sensors with differing price and performance attributes. Hall effect-sensors are typically used to detect fields down to approximately 10−6 Tesla. These sensors are stable, compact, relatively inexpensive and have a large dynamic range. Anisotropic magnetoresistive (AMR) sensors can detect fields down to approximately 10−9 Tesla While these sensors are compact and relatively inexpensive, they are highly prone to drift and have a small dynamic range. Therefore AMR sensors need to be reinitialized frequently using high current pulses. Fluxgate magnetometers can detect fields down to approximately 10−11 Tesla. However these sensors are expensive, bulky and have a relatively small dynamic range. SQUID magnetometers can detect fields down to approximately 10−15 Tesla. They are also expensive with significant operating costs since they require cryogens or a high-power closed-cycle cooling system.
Therefore, there is a need for a surgical navigation system that includes magnetoresistance sensors having a small form factor, excellent signal-to-noise ratio, excellent low frequency operation, lower sensitivity to parasitic inductance and capacitance, lower sensitivity to distortion, and are very low cost to manufacture.